Citric acid has broad application prospects and market demand. For economic reasons, some studies have used less expensive raw sugar and agricultural waste to produce citric acid based on existing strains, while others have been devoted to improved strains. Transformation and selection of production strains is the basis of citric acid fermentation industry, determines the success of the fermentation process and the value of the industrialized production of fermentation products. Although citric acid conversion rate is close to the theoretical level in China, the yield can reach 170 g·L−1, but the fermentation period last a long time, the industrial production fermentation period is generally 72 h, and the industrial fermentation level needs to be improved. Now, the industry is in a very difficult situation and, according to Alvarez-Vasquez's model, citric acid production by Aspergillus niger still has tremendous room to grow, so improvements in citric acid production strains of Aspergillus niger have been the focus of attention for decades.
At present, Aspergillus niger citric acid industrial production strains are obtained by mutagenesis, a large number of studies are still focused on the industrial production strains using methyl methanesulfonate, nitrosoguanidine, Co-ray, UV, such as single or multiple mutagenesis to breed more productive strains. In recent years, Aspergillus niger genome sequencing data has been published, which can construct a comprehensive metabolic network of Aspergillus niger. The use of metabolic engineering to transform Aspergillus niger can purposefully modify cellular metabolic pathways and reduce the huge workload of mutagenesis breeding screening. Based on the available transcriptome data, it was found that GABA pathway has great influence on citrate synthesis. On the one hand, it supplements the succinic acid needed by TCA cycle and on the other hand improves cell acid resistance. It is expected that GABA pathway may be further enhanced by metabolic transformation.